The Effect of Gamma-Irradiation on the Structure and Binding Properties of Amorphous and Partially-Crystalline Geopolymer Matrices

Geopolymers, which are prepared by alkaline activation of alumino-silicate materials, either natural materials such as clays, or industrials by-products such as coal fly ash or slag, are considered as candidates for radioactive waste immobilization due to the combination of metal binding properties with chemical stability and improved mechanical properties. The effects of ionizing radiation on the stability and the performance of these matrices have scarcely been reported. Recent publications showed that gamma-ray irradiation induces a densification of the material due to microstructural modifications in geopolymer and improves the mechanical strength [1-2].

In this study we compared the influence of gamma irradiation on metakaolin based geopolymers of different formulations: high-silica formulations which form yield amorphous microstructure or low-silica formulations which result in a partially crystalline geopolymeric matrix. The effect of sodium nitrate, which is a major component in many nuclear waste streams, on the structure and radiation stability of the geopolymeric product was also evaluated. Samples of the various formulations were irradiated (⁶⁰Co source, 5 MGy) for 6 months under inert conditions.

XRD measurements with PDF (Pair distribution function) analyses indicate an increase in the crystallinity due to the addition of sodium nitrate for the low silica formulation. No clear indication of structural changes due to irradiation was found for all four formulations. FTIR data reveal that the nitrate ions have undergone partial radiolysis to yield nitrite ions.

Nitrate bearing samples had undergone leaching tests which were performed according to the ANSI/ANS-16.1-1986 protocol [3]. The immobilization of Na, nitrate and nitrite ions by the low-silica partially crystalline samples was higher than in their high-silica amorphous counterparts. Furthermore, the immobilization of nitrates was more efficient than that of nitrites. These differences will be discussed in view of the structural data available for the different geopolymer matrices.

References
1) Lambertin, D., et al., "Influence of gamma ray irradiation on metakaolin based sodium geopolymer", Journal of Nuclear Materials, 443, 1-3, 311-315 (2013)

2) Rooses, A., et al., "Encapsulation of Mg-Zr alloy in metakaolin-based geopolymer", Applied Clay Science, 73, 86-92 (2013)

3) ANSI/ANS-16.1-1986, Measurement of the leachability of solidified low-level radioactive wastes by a short-term test procedure, American Nuclear Society 1986.